Iridium anomaly and extraterrestrial component in the clays at ...

Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/125 Comunicações Geológicas (2012) 99, 2, 27-34 ISSN: 0873-948X; e-ISSN: 1647-581X

Iridium anomaly and extraterrestrial component in the clays at the Cretaceous-Paleogene boundary in Denmark, Spain and New Zealand Anomalia de irídio e componente extraterrestre das argilas do limite Cretácico – Paleogénico na Dinamarca, Espanha e Nova Zelândia P. I. Premović1*, B. S. Ilić2 Recebido em 28/09/2011 / Aceite em 11/12/2011

Disponível online em Janeiro de 2012 / Publicado em Dezembro de 2012

© 2012 LNEG – Laboratório Nacional de Geologia e Energia IP

Abstract: The Cretaceous-Paleogene boundary clays at Højerup (Denmark), Caravaca (Spain) and Woodside Creek (New Zealand) show anomalous enrichments of iridium compared with the marine sedimentary rocks. For the average iridium content of 465 ppb of CI carbonaceous chondrite the estimate of CI chondrite proportions in the decarbonated iridium-rich boundary layers, based on the integrated iridium fluencies, is about 25 % at Højerup, 15 % at Caravaca and > 30 % at Woodside Creek. These proportions are most likely too high due to a significant Ir influx from the nearby marine or continental site to these sections.

Keywords: Fish Clay, Caravaca, Woodside Creek, Iridium, Carbonaceous chondrite. Resumo: As argilas do limite Cretácico – Paleogénico em Højerup (Dinamarca), Caravaca (Espanha) e Woodside Creek (Nova Zelândia) mostram enriquecimentos anómalos de irídio comparados com rochas sedimentares marinhas. Para o conteúdo médio de irídio de 465 ppb do condrito carbonáceo CI, a estimativa das proporções condríticas CI nas camadas descarbonatadas ricas em irídio da zona de limite, baseado nas fluências integradas de irídio, é cerca de 25% em Højerup, 15 % em Caravaca e > 30% em Woodside Creek. Estas proporções demasiado elevadas são provavelmente devidas a influxo significativo de Ir a partir das regiões marinhas ou continentais próximas destas secções.

Palavras-chave: Argila, Caravaca, Woodside Creek, irídio, condrito carbonáceo. 1Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, P.O. Box 224, 18000 Niš, Serbia. 2Department of Pharmacy, Faculty of Medicine, University of Niš, 18000 Niš, Serbia. *Corresponding author / Autor correspondente: [email protected]

1. Introduction

In the original paper Alvarez et al. (1980) have first reported anomalously high Ir concentrations in the Cretaceous-Paleogene (KPB) boundary clays at Gubbio (central Italy, Fig. 1), Højerup (the eastern Denmark, Fig. 1) and at Woodside Creek (the northern part of the South Island of the New Zealand, Fig. 1). They proposed an impact of extraterrestrial bolide to explain the elevated Ir content at the KPB. Alvarez et al. also suggested that approximately 60 times the mass of the impactor would have been ejected into the atmosphere as impact dust. Almost simultaneously with Alvarez et al., Smit & Hertogen (1980) reported an anomalous Ir in the boundary clay at Caravaca (southeastern Spain, Fig. 1). Since the Alvarez et al. discovery, the KPB has

been identified and studied at about 345 sites worldwide. Of these, 85 sites, representing all depositional environments, contain an Ir anomaly. In general, Ir and other platinum-group elements (PGE: collectively the elements Ru, Rh, Pd, Os, Ir and Pt) are invariably enriched in the prominent boundary clays. Other trace elements (e. g. heavy metals) are also relatively abundant in these clays.

Many researchers consider that the KPB impactor formed the ca. 180 km crater at Chicxulub (Yucatan Peninsula, Mexico, Fig. 1). It has been suggested that the impactor was a carbonaceous chondrite projectile - probably type CI (Kyte, 1998; Shukolyukov & Lugmair, 1998; Frei & Frei, 2002; Quitté et al., 2003; Trinquier et al., 2006), though it is still unclear whether it was a carbonaceous chondrite or a comet. Indeed, comets are believed to be primitive bodies with a composition like that of carbonaceous chondrites (Gelinas et al., 2004). The anomalous Ir associated, however, with the prominent boundary clay is consistent with the high Ir content typical of most chondritic meteorites and inconsistent with the general proposition of comets. Indeed, a simple calculation shows that in the case of the ice-rich (>70 %) comets the amount of Ir produced by an impact energy for a crater of the Chicxulub size could be less than 0.001 % then that of an asteroid.

In the past many researchers used the iridium concentration and/or integrated amount of Ir to estimate the proportion of chondritic component in the renowned boundary clays. This paper aims to re-examine this method using comprehensive Ir data for the boundary sections at Højerup, Caravaca and Woodside Creek which are available and published by Schmitz (1988). These stratigraphic sections are well preserved and distal (paleodistance: >7000 km) to the proposed Chicxulub impact site. An Ir analysis of the decarbonated KPB samples from Denmark, Spain and New Zealand was carried out by Schmitz using instrumental neutron activation analysis (INAA). Relative error in the precision of the analyses ranges from 5 % to 10 %. Total uncertainties (including accuracy errors) were up to 20 %.

For the sake of completeness, the Ir data related to the distal KPB sections at Agost (Spain), Flaxbourne River (New Zealand), Gubbio (Italy), Bidart (France), El Kef and Aïn Settara (Tunisia), and in Ocean Drilling Program (ODP) Hole 738C (Kerguelen Plateau, southern Indian Ocean) (Fig. 1) are also briefly discussed. Although the sections studied systematically show an Ir abundance anomaly, they differ from one another because local sedimentation conditions under a strong continental influence. Throughout this paper we make five reasonable postulates: (a) Ir

Artigo original Original article

28 P.I. Premović et al. / Comunicações Geológicas (2012) 99, 2, 27-34

is wholly located in the non-carbonate fraction of the boundary clays studied, i. e. the carbonate fraction of this section is essentially an Ir diluents; (b) the Ir content of the carbonaceous chondrites range from 406.0 ppb to 849.4 ppb (Table 1); (c) the average content of Ir in CI chondrites is 465 ppb (Table 1); (d) all Ir found in the boundary clays originated from the carbonaceous chondrite (CC) impactor, especially of type CI; and (e) assumed density of the boundary clays is about 2 g cm-3.

Fig.1. Geographic location of studied KPB clays. Fig.1. Localização geográfica das argilas KPB estudadas.

2. Results and Discussion

Fish Clay. The lowermost Danian Fish clay Member of the Rødvig Formation near the village of Højerup is a classic marine KPB section. The lithology of the Fish Clay within this boundary section characterizes three distinctive layers (from bottom to top): a 3 cm thick (mainly black-to-dark) marl (hereinafter BH) with 0.5 cm-thick basal red (goethite-rich) sublayer, grey-to-brown marl and a light-grey marl (Fig. 2). The red sublayer is underlain by (the latest) Maastrichtian bryozoan-rich limestone (chalk) whereas the top marl is overlain by (early) Danian Cerithium limestone (Schmitz, 1988; Christensen et al., 1973; Schmitz, 1985; Elliott, 1993; Surlyk et al., 2006). Geochemical studies show that the Ir profile (on a whole rock basis) across the Fish Clay column is characterized by a maximum just above the base of BH with an upward gradual decrease (tailing-off) from its maximum (e. g. Schmitz, 1985). This gradual tailing off indicates that the Fish Clay represent a relatively continuous and complete section. The BH is considered to constitute the main part of the boundary section, since it contains more than 95 % of the total Ir in the Fish Clay (Premović, 2009). The mineralogy of BH is comparatively simple, smectite and authigenic (mainly biogenic) calcite being the principal components. Geochemical evidence indicates that the BH was deposited under strong anoxic conditions but the red sublayer under strong oxic sedimentation conditions (Premović, 2009).

Premović et al. (2000) inferred that the BH was deposited in a shallow marine <100 m water depth at Stevns Klint within an interval of about 40 years. Wendler & Willems (2002) considered that this layer represents the first decades or centuries of deposition following the KPB impact event. Thus, the average sedimentation rate in the BH is probably about 0.3 mm per year.

As pointed out above, the Ir enrichment of the KPB clays is generally regarded as indicative of the presence of a CC component. Table 1 shows that on average marine sedimentary rocks usually contain the Ir bellow 1 ppb (Table 1) while

carbonaceous chondrites typically contain 406.0 - 849.4 ppb Ir (Table 1). So, a little addition of the CC component would be necessary to critically increase the concentration of Ir in a marine sedimentary rock. Thus, Ir is a very sensitive means of examining the CC contribution to the marine KPB clays such as the Fish Clay. Some authors suggested that Ir in the Fish Clay was sourced by the Ir from seawater (Goldberg et al., 1986). However, the average Ir concentration in the decarbonated BH (ca. 100 ppb, see below) represents an enrichment factor of about 1017 compared to seawater (ca. 2×10-15 ppb, Table 1). Moreover, according to these authors, the residence time of Ir in the oceans is about 1 million years. Thus, excess of Ir in the BH could not, therefore, have been derived from the sea reservoir.

Table 1. Concentrations of Ir [ppb] for carbonaceous chondrites, marine sediments and seawater.

Tabela 1. Concentrações de Ir [ppb] para os condritos carbonáceos, sedimentos marinhos e água do mar.

Fig.2. Expanded lithological log of the Fish Clay at Højerup based on Surlyk et al. (2006). Fig.2. Log litológico expandido das argilas em Højerup baseado em Surlyk et al. (2006).

Kyte et al. (1985) estimated that the extraterrestrial component

in the basal 1 cm part of BH from the measured Ir concentration (47.4 ppb) is about 10.5 %, corrected for about 20 % carbonate content. According to Grieve (1997), chondrite-normalized relative abundance of Ir indicates that the Fish Clay (on a whole-rock basis) contains an admixture of about 10 % chondrite. Trinquier et al. (2006) have shown that Cr isotopic signature of BH represents a mixing of the CC matter with terrestrial material in a ratio up to 6.8 %. Recently, Osawa et al. (2009) estimated from the measured Ir concentration (29.9 ppb) that the extraterrestrial material in the basal part of BH was diluted to 1/19 (on a whole-rock basis), assuming that Ir concentration of CC is 470 ppb. Of note, that an impactor mass fraction globally dispersed after the Chicxulub impact ranges between 22 % (Alvarez et al., 1980) to 50 % (Vickery & Melosh, 1990).

Ir anomaly and meteoritic contribution 29

The INAA data (Schmitz, 1988) for Ir in the carbonate-free fraction across the Fish Clay are presented in Fig. 3a. This profile shows that the peak concentration of Ir in the decarbonated BH is about 120 ppb. Using these data, we estimate that the decarbonated BH is derived from about 6 - 30 % CC (Fig. 4a); the CI chondrite proportion is about 22 % for an average Ir concentration of about 100 ppb.

Fig.3. Concentration profiles of Ir (on a carbonate-free basis) in the Cretaceous-Paleogene boundary clays at: (a) Højerup (Fish Clay), (b) Caravaca and (c) Woodside Creek. The samples were analyzed with instrumental neutron activation (INAA). Relative error in the precision of the analyses ranges from 5 % to 10 %. Total uncertainties (including accuracy errors) were up to 20 % (Schmitz, 1988). Fig.3. Perfis de concentração de Ir (numa base sem carbonato) nas argilas do limite Cretácico-Paleogénico em: (a) Højerup (Fish Clay), (b) Caravaca e (c) Woodside Creek. As amostras foram analisadas com activação neutrónica instrumental (INAA). O erro relativo na precisão das análises varia entre 5 % a 10 %. As incertezas totais (incluindo erros de incerteza) chegaram até 20 % (Schmitz, 1988).

A better measure of an Ir anomaly in the boundary clay is the integrated Ir fluence. For the BH, the fluence is estimated at about 305 ng cm-2 after integration in the decarbonated segment from 0.5 - 2.0 cm (Fig. 3a). (For comparison, an estimate of the Ir fluence for the global impact deposit for the KPB is about 40 - 55 ng cm-2: Kyte, 2004). This value corresponds approximately to about 0.35 - 0.75 g cm-2 CC. These estimates correspond to about 15 - 30 % CC; the CI contribution is about 25 %. Such a significant proportion of CC is, however, not supported by any mineralogical evidence. Indeed, if initially the precursor material of BH contained a high percentage of the ejecta fallout, then diagenetic alteration would have left a residue with high concentrations of the impact markers. To our knowledge, the part of BH above the red sublayer contains no altered meteoritic fragments (Bauluz et al., 2000) and only about 0 - 6 shocked quartz grains per gram (Morgan et al., 2006). On the other hand, a simple calculation indicates that the underlying red sublayer (with 15. 2 ppb of Ir, Fig. 3a) contains as low as about 3.3 % of type CI chondrite material. In sharp contrast, this sublayer is abundant (about 10 wt %) with the well-preserved impact-related goethite-rich microspherules (Schmitz, 1985; Graup et al., 1992) and with nano-size goethite grains (Bauluz et al., 2000; Wdowiak et al., 2001) interpreted as altered meteorite fragments (Bauluz et

al., 2000). About 29 - 33 shocked quartz grains per gram are also identified in this layer (Morgan et al., 2006). Of note, the total Ir flux of the carbonate-free Fish Clay is about 615 ng cm-2 which corresponds to about 22 % the type CI chondrite. For the average sedimentation rate in the BH of 0.3 mm yr-1, the average accumulation rate of Ir in the BH is ca. 3 ng cm-2 yr-1; the average accumulation rate is about 6.5 µg cm-2 yr-1 CI chondrite.

Fig.4. The CC contributions for the decarbonated boundary clays at (a) Højerup (Fish Clay), (b) Caravaca and (c) at Woodside Creek. The lowest ( ) and highest ( ) CC contributions are based on the Ir concentrations of the carbonaceous chondrites (see text). Fig.4. As contribuições CC para as argilas descarbonatadas do limite em (a) Højerup (Fish Clay), (b) Caravaca e (c) em Woodside Creek. As contribuições CC mais baixa ( ) e mais elevada ( ) baseiam-se nas concentrações de irídio dos condritos carbonáceos (ver texto).

Our estimation (judging from the Ir concentrations and integrated Ir fluence) may substantially underestimate the actual CC contribution to the decarbonated BH. There are two reasons for this. First, anomalous amounts of Ir in the decarbonated Fish Clay display a gradual decrease from the BH upwards but this metal is still relatively elevated (up to near 5 ppb) in the light-gray marl (Fig. 3a). These stratigraphically extended enrichments of Ir may be explained only partly by the redox-controlled remobilization (Colodner et al., 1992). Second, according to Peucker-Ehrenbrink & Hannigan (2000), between about 40 % and 80 % of the initial Ir budget could have been lost due to surficial weathering of black shales (such as the BH) within roughly 13, 000 years. Thus, because of the assumed remobilization and/or weathering loss of the Ir, the original proportion of CC in the carbonate-free part of BH actually could be even considerably higher than presented above. The more detailed discussion of this issue is beyond the scope of this paper.

Assuming that ejecta fallout (with an average density of about 2 g cm-3) contained 50 % of the CC matter then the BH portion in question would contain about 30 - 60 % of this (mostly diagenetically altered) fallout. The CC matter (and associated Ir) could be deposited in the Fish Clay by the direct airborne ejecta fallout settling through the seawater column or they were transported from the nearby marine or continental site to the Fish


Clay bed. Kyte et al. (1985) reasoned that Ir (and other siderophiles) in the basal 1 cm thick part of BH is only representative of primary Ir derived from the initial ejecta fallout deposited during short time (a few days, months or years) after the impact. According to these researchers, Ir above this layer is probably secondary in origin and transported from the original nearby site, which increased the primary Ir values. Wolbach et al. (1988) proposed that secondary Ir is ejecta material eroded from marine elevated sites to topographic lows already containing some primary fallout. Premović (2009) has suggested that Ir in the Fish Clay was probably fluvially transported from the soil on adjacent land and redeposited in a shallow marine basin at Højerup. This author argued that a predominant part of Ir in the BH ultimately came from the CC material associated with ejecta fallout covering nearby coastal soil. Indeed, the gradual decrease of Ir upsection across the Fish Clay indicates the single input of Ir probably associated with detrital sedimentation, consisting of smectite, silt and humic materials (Premović, 2009, and references therein). Consequently, the overestimated proportion of the CC matter in the decarbonated BH is probably due to the high Ir (detritus-associated) input from the marine or continental site.

Anomalous Ir concentration (on a whole-rock basis) is not limited to the Fish Clay but extends about 1 m above and below the BH (Rocchia et al., 1987). They estimate that the duration of the enhancement represents about 104 years. This may indicate an extended contribution of Ir derived from an extraterrestrial source other than the predominant and virtually instantaneous source of Ir of the KP impactor. It is also possible that extension of Ir enrichment originate from a relatively long terrestrial source, e. g. Deccan Traps volcanism at the end of the Cretaceous period which lasted millions of years. The third explanation is that this tail represents postdepositional redistribution of Ir out of its original peak position. This topic is, however, out of scope here. The Caravaca boundary section. The sections at Agost, Caravaca and El Kef are among the most continuous and complete marine sections for the KPB transition. The Caravaca and Agost boundary sections are located in the Betis Cordilleras (southerneast Spain). The KPB section at Caravaca consists of a ca. 1 cm-thick Ir-rich dark marl (BC) overlain by a grey-to-brown marl, Fig. 3b. A basal 2-3 mm thick red sublayer of BC enriched with Ir (ca. 47 - 57 ppb: Tredoux et al., 1989) is marking the KPB. The clay association of these marls is dominated by dioctahedral smectite (Ortega-Huertas et al., 1995). According to these researchers, this smectite results from (a) the alteration of marine volcanic rocks or (b) the erosion of continental soil. Terminal Maastrichtian–basal Paleogene marls at Caravaca were deposited in a middle bathyal environment (<500 m depth) (Smit, 1999). Abundant presence of goethite in the red sublayer indicates that its deposition probably occurred under well-oxygenated conditions.

The distribution of Ir in the carbonate-free fraction across the boundary section at Caravaca is given in Fig. 3b. This profile is based on the INAA measurements performed by Schmitz (1988). The Ir values reach a profound peak concentration of about 110 ppb in the decarbonated red sublayer. The main Ir peak has tails both upsection and downsection, but the later is much sharper than in the Fish Clay. As previously stated, the upward tail feature infers a relatively continuous and complete section. Using his concentration data, we estimate that the decarbonated BC is derived from ca. 5 - 27 % CC (Fig. 4b); the CI chondrite contribution is about 13 % for an average Ir concentration of about 60 ppb.

The integrated Ir fluence of BC is estimated at about 100 ng cm-2. This value corresponds to 0.20 - 0.40 g cm-2 CC. These in

turn correspond to about 10 - 20 % CC; the CI chondrite contribution is about 15 %.

Assuming that ejecta fallout (with an average density of about 2 g cm-3) contained 50 % of the CC matter then the BW portion in question would contain about 20 - 40 % of this (mostly diagenetically altered) fallout. As previously stated for the BH, a high percentage of the ejecta fallout would have left a residue after diagenesis with high concentrations of the impact indicators in the BC. The thin red sublayer contains numerous presumably impact-derived goethitic microspherules (ca. 10 % of the total weight: Schmitz, 1988), shocked zircons and Ni-rich spinels. About 19 - 38 shocked quartz grains per gram were identified in the BC (Morgan et al., 2006).

By analogy with the Fish Clay, we assume that Ir in the BC was probably derived from carbonaceous chondritic component of ejecta fallout accumulated on nearby adjacent land or marine area. Indeed, the major carrier of this Ir was probably the smectite which is most likely of detrital origin (Ortega-Huertas et al., 1995). The decarbonated boundary section at Caravaca is characterized with a similar stratigraphically extendeded enrichments (<5 ppb) as the decarbonated Fish Clay, Fig. 3b. Moreover, Rocchia et al. (1987) reported the Ir-rich interval at Caravaca also contains above-background concentrations of Ir (on a whole-rock basis) over 1.5 m of interval. The Woodside Creek boundary section. The KPB section at Woodside Creek is represented by a reddish (up to 1 cm thick) clay-rich layer (BW) and overlying dark to grey-to-brown marls. These layers were probably deposited in a shallow marine <500 m water depth (Morgan et al. 2006) probably under well-oxygenated conditions. BW is underlain by the latest Maastrichtian marl, Fig. 3c.

The INAA data for Ir (Schmitz, 1988) in the decarbonated fraction of the boundary section at Woodside Creek are plotted in Fig. 3c. The peak concentration of Ir of 460 ppb is located in the carbonate-free BW which is one of the highest measured to date for any KPB interval. Using Schmitz’s data, we estimate that in the decarbonated BW is derived from about 60 % up to bizarre 115 % CC (Fig. 4c); the type CI chondrite input averages about 100 %.

The Ir anomaly appears to be much sharper upward and downward than at the Danian (Fig. 3a) and Spanish (Fig. 3b) sites. Moreover, the decarbonated Ir-rich section at Woodside Creek is characterized with high Ir enrichments (20-30 ppb) up to about 5-6 cm above the BW, Fig. 3c.

Brooks et al. (1984) reported that a carbonate-free boundary layer (0.8 cm thick) at another site of Woodside Creeks contains about 153 ppb of Ir. This value corresponds to about 20 - 40 % CC; the CI chondrite contribution is about 30 %.

Assuming that ejecta fallout (with an average density of about 2 g cm-3) contained 50 % of the CC matter then the BH portion in question would contain about >40 % of this (mostly diagenetically altered) fallout. As in the case of BH, a high percentage of the ejecta fallout would have left a residue after diagenesis with high concentrations of the impact indicators in the BW. To express precisely, the BW contains numerous goethite-rich and organic microspherules (Schmitz, 1988); and, about 2 - 8 shocked quartz grains per gram (Morgan et al., 2006). Anomalous extraterrestrial Ir is also mostly associated with this layer, Fig. 3c.

The Agost boundary section. The Agost boundary section is similar to the neighbouring Caravaca section in lithology, geochemistry and depositional history. As at Caravaca this section is comprised of a dark (about 6-cm-thick) clay with a basal 2-3 mm-thick red goethite-rich sublayer, Fig. 5. The dark


clay layer is underlain with the latest Maastrichtian marl and overlain by the grey-to-brown marl (Molina et al., 2005, and references therein). This layer is deposited in the middle bathyal environment, 600-1000 m deep (Coccioni & Galeotti, 1994).

Fig.5. (a) The Ir concentration profile (on a carbonate-free basis) across the Cretaceous-Paleogene boundary section at Agost and (b) the CC contributions for the decarbonated boundary section at Agost: lowest ( ) and highest ( ) CC contributions (see above). Fig.5. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite Cretácico-Paleogénico em Agost e (b) as contribuições CC para a secção descarbonatada do limite em Agost: contribuições CC mais baixa ( ) e mais elevada ( ) (ver acima).

Smit (1990) analyzed Ir (on a whole-rock basis) across the dark clay. Using his Ir and carbonate content data, we computed Ir concentrations in the non-carbonate (clay) fraction. These calculated concentrations are plotted vs. stratigraphic height in Fig. 5a. The results show that the highest Ir enrichment (ca. 30 ppb) is in the red sublayer with an upward gradual decrease from this peak. We estimate that the decarbonated sublayer contains about 4.0 - 7.0 % CC (Fig. 5b). The red sublayer contains mineralogical evidence of extraterrestrial impact such as Ni-rich spinels and diagenetically altered K-feldspar or Fe oxide microspherules (Smit, 1990).

The Flaxbourne River boundary section. Beside at Woodside Creek, the KPB has been identified in over 20 stratigraphic sections in the New Zealand region and one of them is Flaxbourne River (Hollis et al., 2003). Despite the lesser Ir enrichment in the Flaxbourne section (21 ppb) compared to Woodside Creek (54-100 ppb), the Flaxbourne section is considered to be more complete (Strong, 2000; Hollis et al., 2003). The dark-to-black boundary clay (average in thickness about 2 cm) in this section is a calcareous limestone which is overlain by a dark limestone. This clay in turn is underlain by the latest Maastrichtian marl, Fig. 6.

The INAA data for Ir (Strong et al., 1987) in the decarbonated fraction of the boundary section at Flaxbourne River are plotted in Fig. 6a. The peak concentration of Ir of about 21 ppb is located in the carbonate-free boundary clay; more than about 85 % of the total Ir is distributed within the Ir-rich interval of -2 cm to +6 cm. Using their data, we estimate that the decarbonated

boundary clay contains from about 0.25 - 5.0 % CC (Fig. 6b). However, the decarbonated Ir-rich interval at Flaxbourn River is characterized with another high Ir peak (ca. 20 ppb) about 1.5 cm below the boundary clay, Fig. 6a.

Fig.6. (a) The Ir concentration profile (on a carbonate-free basis) across the Cretaceous-Paleogene boundary section at Flaxbourne River and (b) the CC contributions for the decarbonated boundary section at Flaxbourne River: lowest ( ) and highest ( ) CC contributions (see above). Fig.6. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite Cretácico-Paleogénico em Flaxbourne River e (b) as contribuições CC para a secção descarbonatada do limite em Flaxbourne River: contribuições CC mais baixa ( ) e mais elevada ( ) (ver acima).

The Gubbio boundary section. The classical Bottaccione boundary shales near Gubbio originated at a deep marine bathyal basin with a water depth over 1500 m (Kuhnt, 1990). A stratigraphic profile for the basal part of these shales is shown in Fig. 7. We roughly estimate that the CC matter in their decarbonated fraction is <2 %. This estimation is based on the peak Ir concentration (ca. 7 ppb: Crocket et al., 1988). These researchers also found that the enrichment of Ir (on a whole-rock basis) at Gubbio is not confined to the boundary shales but extends approximately 2 m above and below the boundary level, corresponding to 3.5 million years. According to Montanari (1986), the boundary section at Gubbio bears evidence of widespread contamination from the surrounding soil. The Bidart boundary section. The boundary section at Bidart is characterized by a sharp contrast between a marl of the latest Maastrichtian and a 6-cm-thick layer of dark boundary clay with a 2-mm-thick basal (iron-rich) red sublayer, Fig. 8a. This clay of the lowermost Danian is overlain by brownish clay. The whole interval is deposited in upper-middle bathyal depth (Alegret et al., 2004). The red sublayer is marked by an Ir anomaly (Rocchia et al., 1987) and the presence of Ni-rich spinels (Robin & Rocchia, 1998). In Fig. 8a, we plot a Ir distribution(on a carbonate-free basis) across the boundary clay using the Ir and carbonate content data reported by Rocchia et al. (1987). We roughly estimate that the carbonate-free fraction of the red sublayer contains <2 % CC (Fig. 9a).


Fig.7. Expanded lithological log across the Cretaceous-Paleogene boundary section at Gubbio (adopted from Premović, 2009). Fig.7. Log litológico expandido ao longo do limite Cretácico-Paleogénico em Gubbio (adaptado de Premović, 2009).

Fig.8. The Ir concentration profile (on a carbonate-free basis) across the Cretaceous-Paleogene boundary section at: (a) Bidart, (b) El Kef and (c) Aïn Settara. Fig.8. O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite Cretácico-Paleogénico em: (a) Bidart, (b) El Kef e (c) Aïn Settara.

El Kef boundary section. The base of the El Kef boundary section in the central Tunisia has been officially designated as the boundary global stratotype section and point (GSSP) for the KPB (Cowie et al., 1989; Molina et al., 1996). At El Kef the boundary

clay is 55–65 cm thick, with the 2–3 mm (almost carbonate-free) red (goethite-rich) sublayer (Keller et al., 1995); this sublayer contains 5 - 10 % of the total Ir flux and most of Ni-rich spinels (Robin et al., 1991). A stratigraphic column of basal part of this clay is presented in Fig. 9b. Smectite is the main component of the boundary clay at this site (Ortega-Huertas et al., 1998). This clay passes upward into marl of the early Danian (Molina et al., 2006). The Ir maximum of 4 ppb in the red sublayer (Robin et al., 1991) corresponding roughly to <1.5 % (Fig. 9b). The Aïn Settara boundary section. This section (about 50 km far from the El Kef site) is similar to the El Kef section in lithology, geochemistry and deposition history (Dupius et al., 2001); a stratigraphic column of basal part of this section is shown in Fig. 8c. According to these researchers, (almost) carbonate-free jarositic red sublayer is a few mm thick contains 3 ppb of Ir; this corresponds roughly <1 % of the CI chondrite matter. We roughly estimate that the red sublayer contains <1 % CC (Fig. 9c).

Fig.9. The CC contributions for the decarbonated boundary section at: (a) Bidart, (b) El Kef and (c) Aïn Settara: lowest ( ) and highest ( ) CC contributions (see above). Fig.9. Contribuições CC para a secção descarbonatada do limite em: (a) Bidart, (b) El Kef e (c) Aïn Settara: contribuições CC mais baixa ( ) e mais elevada ( ) (ver acima).

ODP Hole 738. Schmitz et al. (1991) analyzed Ir (on a whole-rock basis) across a 1m-thick-clay rich boundary section in ODP Hole 738. They found that Ir concentrations are the highest (18 ppb) in the dark boundary clay (lamina) a few centimeters above the base of the clay-rich layer. Using their Ir and carbonate content data, we computed Ir concentrations in the non-carbonate (clay) fraction. These calculated concentrations in Fig. 10a are plotted vs. stratigraphic height. The results show that the highest Ir enrichment (ca. 60 ppb) is in the dark lamina. The Ir concentrations decrease relatively sharply for about 1.5 cm upsection and then remains nearly constant over a plateau of tens of centimeters. We roughly estimate that the decarbonated fraction of the dark lamina contains between 5 % and 15 % CC (Fig. 10b). No shocked quartz or impact-derived microparticles were reported to be found in the boundary section. Schmitz et al. (1991) concluded that the clay fraction of this section studied is predominantly derived from locally derived material.


Fig.10. (a) The Ir concentration profile (on a carbonate-free basis) across the Cretaceous-Paleogene boundary section at ODP Hole 738 and (b) the CC contributions for the decarbonated boundary section at ODP Hole 738: lowest ( ) and highest ( ) CC contributions (see above). Fig.10. (a) O perfil de concentração de Ir (numa base sem carbonato) ao longo do limite Cretácico-Paleogénico no buraco 738 do ODP e (b) as contribuições CC para a secção descarbonatada do limite no buraco 738 do ODP: contribuições CC mais baixa ( ) e mais elevada ( ) (ver acima).

In summary, the Ir content of marine boundary clays may be influenced by various sedimentary and geochemical factors (e.g., erosion, redeposition, geochemical remobilization, and weathering). The extent to which these factors operate is dependent on the particular sedimentary site and the immediate seafloor environment. Consequently, Ir may have been concentrated or diluted in the boundary clay during sedimentation and diagenesis, and afterwards. This makes ambiguous any attempt to assess the contribution of the impactor Ir to the Ir enrichment of the boundary clay or to use global fluence of Ir to estimate the size of the impactor. The same ambiguity goes for other PGE. In addition, in order to identify chondritic elements in the boundary clay the researchers usually present their trace element data normalizing first to Ir and then to CI chondrites. It is clear from the above consideration that this method is also rather ambiguous.

3. Conclusions

Estimates based on the integrated Ir amount show that the decarbonated Ir-rich KPB layers at Højerup, Caravaca and Woodside Creek appear to contain respectfully as much as 25 %, 15 % and >30 % of the CI matter for the average Ir content of 465 ppb CI chondrite. These percentages are most probably an overestimation due to the high Ir input from the nearby marine or continental site, which presumably increases primary Ir values in these clays.

Acknowledgements

We are grateful to Dr. Adam Dangić for supplying constructive comments. We also thank to Dr. Telmo Santos for his understanding

and endless patience for our effort to make this report as complete as possible.

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